Cellular telecommunications networks characteristically provide “cells” of radio communication coverage between communication devices (which are typically mobile) and a core network (with a “downlink” from core network to communication device and an “uplink” in the opposite direction).
Various radio access technologies (RATs) are implemented: currently digital cellular networks are the most common and these are loosely classed as second generation (2G), third generation (3G), fourth generation (4G), etc. technologies according to whether the RAT achieves effective data communications that meet increasingly challenging requirements. Included in the 4G standards are the third generation partnership project (3GPP) long term evolution (LTE) and LTE-Advanced (LTE-A), which correspond to release 10 and later of the 3GPP standards. Legacy 1G systems used analogue radio transmission of mainly voice data, whereas 2G systems utilise digital radio transmission. Earlier 2G wireless communication systems used only circuit switching, but later 2.5G systems used both circuit and packet switching, as do 3G systems. 4G and LTE/LTE-A technologies are solely packet-switched, with circuit-switched fall-back to earlier technologies.
The way that the information is communicated on the radio waves differs between the different generations. 3G technologies use Code Division Multiple Access (CDMA) modulation and a hybrid core network that treats data and voice differently. CDMA employs spread-spectrum technology (transmission bandwidth greater than frequency content of original information) and a coding scheme where each transmitter is assigned a code, allowing several transmitters to send information simultaneously over a single communication channel. 4G technologies use Orthogonal Frequency Division Multiple Access (OFDMA) modulation on the downlink (SD-FDMA on the uplink) and an Internet Protocol (IP) core network to communicate both data and voice. OFDM is a frequency-division multiplexing scheme in which a large number of orthogonal sub-carrier signals carry data on parallel data streams or channels. Each sub-carrier is modulated at a low symbol rate giving a total data rate similar to a conventional single-carrier modulation scheme in the same bandwidth. OFDMA and SD-FDMA use very similar computational structures. The LTE wireless interface is incompatible with 2G and 3G networks, so it is operated on a separate wireless spectrum.
To ensure effective coverage of a large geographic area, a plurality of cells are provided by respective network nodes, referred to in 2G as base transceiver stations (BTS) or base stations, referred to in 3G as NodeBs and referred to in 4G as evolved nodeBs or eNodeBs or eNBs. Network nodes such as eNodeBs are associated with one or more antenna arrays which in turn establish respective “cells”. A network node may be referred to as a “cell site”, which is different from the cell per se (i.e. the coverage area). In the simple case of an omnidirectional antenna being associated with each network node, each network node has a corresponding associated cell corresponding to a geographical coverage of the radio signal from the antenna. However, some network nodes comprise multi-directional antenna, with each antenna covering a so-called “sector” of a cell. For example, for a tri-directional antenna, each antenna may cover a 120° sector of a cell. The cells are logical constructs intended to simplistically represent a complex object corresponding to a geographical area covered by a radio antenna. The cells correspond to coverage areas where it is statistically likely for a UE to connect to the corresponding network node (i.e. transmitting antenna). For a sectored cell a circle is drawn to encompass the pie shape corresponding to an individual sector and the cell centroid according to the present technique is the centre of that circle.
The network nodes are controlled at least in part by other entities in the core network known as controllers. In 2G technologies the controllers are base station controllers (BSC); in 3G the controllers are radio network controllers (RNC); and in 4G LTE and LTE-A technologies there is a flatter architecture without a BSC or an RNC and the eNodeB (4G network node) includes radio resource management functionality. In LTE and LTE-A, the eNodeB is connected to an Evolved Packet Core (EPC) via a mobile management entity (MME) for control plane signalling and a Serving Gateway (S-GW) for user plane data. In wireless communication systems the BTS, NodeB or eNodeB has a first (radio) interface to the mobile station (or user equipment UE) and a second interface to the core network.
The interface between the BTS/NodeB/eNodeB and the core network is known as the backhaul interface or backhaul link. In 4G networks there is a single Ethernet cable connecting an eNodeB to an IP backhaul network. In LTE/LTE-A the backhaul link comprises both an “S1” interface which links the eNodeB to the evolved Packet Core and an “X2” interface that allows signalling between different eNodeBs. Both signalling and application data are communicated on the backhaul link. Currently, the physical backhaul interface is likely to be a Time Division Multiplexing (TDM) interface or an Ethernet interface. The X2 interface is not present in GSM (2G) or WCDMA (3G). In LTE, the X2 interface is only used for direct handovers between neighbouring eNodeB. For such direct handovers, the destination eNodeB co-ordinates with the S-GW/MME to shift traffic (that is being sent over the X2 interface during the handover) from the source eNodeB to the S1 interface for the destination eNodeB. There can be a large number of neighbours for each eNodeB (e.g. up to 32) and the set of neighbours for a given eNodeB is unique and dynamic such that a given eNodeB may have a set of radio neighbours that changes over time.
It is known to estimate the location of a mobile terminal at a given time using a cell identifier (ID) of the cell that the mobile terminal is connected to at that time. However, this method of location estimation has limited accuracy because it estimates the same location for all mobile terminals connected to the same cell i.e., the centre of the predicted coverage area of the cell commonly referred to as the cell centroid. It is also known to estimate a home location for a mobile terminal to be the cell centroid having the highest amount of phone activity overnight. The centroid is the geometric centre of a shape such as a circle or a polygon and corresponds to the arithmetic mean of all points in the shape.
There is a requirement to provide an efficient yet more accurate system for estimating a static location such as a home location for a mobile terminal.